This invention relates to optical transmission elements in general and more particularly to an improved fiber optic transmission system which includes a multi-layer fiber comprising a core and sheath surrounded by a protective jacket.
Fibers for optical transmission made from a core having a higher index of refraction and sheath with a lower index of refraction are known in the art. In fibers of this nature the light introduced into the core is reflected back into the core by total reflection at the boundary surface between the core and sheath. Seen optically and geometrically, the light travels through the fiber in a zigzag fashion. Developments over recent years have led to fiber optic transmission elements with two dB of attenuation per kilometer of length. Such developments brings the transmission of information using optical wave guides into the realm of possibility. In addition to the unavoidable attenuation in the material of the core, however, further losses occur in fiber optics due to the fact that a portion of the light is transported in the sheath surrounding the core despite the total reflection. More specifically, there is a cylindrical layer of several micrometers thickness in which light travels. It has been recently discovered that the thickness of this layer depends on the optical coupling of the succeeding layers and thus on their optical characteristics. i.e., the index of refraction and coefficient of absoption. This component of light which travels through the sheath cannot be neglected, particularly where light is being transmitted over long distances without intermediate amplification. Thus, it becomes necessary to keep light losses in the sheath as low as possible. If the fiber is bent and forms a curve, the share of the light in the sheath is further increased with decreasing radius of curvature. This results from the fact that the angle of total reflection is more nearly approached and may partially be exceeded. To avoid higher attenuation, sheath light must not get lost. Such light can get lost through radiation into the environment, e.g. into the air, if the angle of total reflection prescribed therefor is exceeded. Such can happen, particularly where the outer sheath surface is irregular. Further losses can occur if a protective jacket surrounding the fiber consists a material having a higher index of refraction than the sheath material with the protective jacket directly in optical contact with the sheath surface. All plastic materials used for protective jackets have an index of refraction which is larger than that of the sheath. Under such circumstances an angle of total reflection no longer exists, i.e. all the light reaching this boundry surface leaves the sheath and is lost.
In view of these problems the need for an improved design for an optical transmission element in which attenuation due to radiated or absorbed sheath light is avoided becomes evident.